A photomicrograph shows an X chromosome at left, alongside a shrunken Y chromosome. The Y chromosome is passed down exclusively from father to son and can serve as an indicator of male-line human diversity. (University of Arizona)

Scientists say an African-American male’s odd genetic signature suggests that the human Y chromosome’s lineage goes back further in time than they thought — perhaps due to interbreeding with other populations such as Neanderthals.

The study focuses on the analysis of a DNA sample that was obtained from an African-American living in South Carolina and submitted to the Genographic Project, a National Geographic effort aimed at mapping human origins and migration. The funny thing about this sample is that it didn’t match up with any of the previously known genetic signatures for the Y chromosome, which is passed down from father to son.

“Nobody expected to find anything like this,” Hammer said in a news release.

A team led by Fernando Mendez, a researcher in Hammer’s lab, analyzed more than 240,000 DNA base pairs on the African-American’s Y chromosome. A comparison of the differences between the mystery genetic signature and previously known signatures led the team to conclude that the most recent common ancestor for the entire group lived about 338,000 years ago.